1. Field of the Invention
This invention generally relates to integrated circuit (IC) and liquid crystal display (LCD) fabrication and, more particularly, to a system and method for laser annealing using a digital light valve gating mechanism.
2. Description of the Related Art
The fabrication of electronic devices such as integrated circuits and thin film transistors requires many process steps, including deposition, etching, annealing, crystallization, and others. Each of these steps requires one or more energy sources applied to the substrate, and/or neighboring material, to complete the process. Many of these processes also require a photolithography step to isolate the area to be processed. Photolithography and masking steps are very costly manufacturing processes.
FIGS. 13A and 13B illustrate, respectively, a conventional laser crystallization lateral growth process and a mask used in support of such a process (prior art). A laser source is conventionally used to crystallize amorphous silicon (a-Si) on temperature sensitive substrates, such as glass or plastic, in the manufacture of LCDs. A mask permits selected areas of Si to be heated to melting, without degrading the underlying substrate. One conventional process, known as Sequential Lateral Solidification (SLS) or Laser-Induced Lateral Growth, uses a mask to sequentially expose adjacent strips of a-Si to laser light, crystallizing the a-Si into polycrystalline Si (poly-Si). The poly-Si is used to fabricate electrical devices, such as TFTs for display applications. Alternately, polysilicon materials can be used in the manufacture of mechanical and photonic devices.
The prior methods for SLS or lateral crystallization use a mask to form the laser beam into beamlets that are directed on the substrate. These beamlets are effectively scanned in a stepping motion across a substrate producing a laterally crystallized polysilicon material. This polysilicon material on substrate is typically used to fabricate TFT array backplanes for display products. There are two major problems with this method. The material electrical properties of transistors, with channels parallel to the scan direction of the laser beam, are of a very high quality. However, the material electrical properties of transistors, with channels running perpendicular to the scan direction of the laser beam, are only approximately 50% of those with channels parallel to the scan direction. Thus, large non-uniformities in the characteristics of thin film transistors that are perpendicular to each other. This non-uniformity results in a low quality display, due to the non-uniformity of discreet pixel TFTs and non-uniformities between TFTs making up circuit elements.
Further, the masks used to form the laser beamlets are expensive and susceptible to damage over time. In addition, a manufacturer may be required to use many masks to process different types of crystallization. For example, masks with different slit widths are used to promote different crystallization results. It is also known to use a sequential series of different masks. Specialized masks combining different slit widths and directions are also used for particular applications. All these specialized masks, in turn, require unique reticles and mask fabrication processes. Thus, for particular crystallization applications, the expenses associated with the initial cost of the masks, and the process steps of changing masks, can be prohibitive.
It would be advantageous if selective areas of Si could be crystallized without using a mask.
It would be advantageous if the selective areas of crystallization could be dynamically modified without making a corresponding change to a mask.
It would be advantageous if laser light could sequentially expose regions of a substrate without the necessity of a mask or photoresist step.